1. Field of the Invention
The present invention relates to a liquid crystal shutter, and a printhead provided with a liquid crystal shutter.
2. Description of the Related Art
An electronic image captured by a digital camera, for example, can be printed on an ordinary paper based on the digital data by inkjet or thermal transfer. It is also conceivable to print an image as digital data on a photosensitive film by a photosensitive system. In the photosensitive system, an image is formed on a photosensitive film by exposing the photosensitive film to light followed by developing the film using a photo printhead. A typical photo printhead is provided with a liquid crystal shutter for selectively passing or blocking light travelling from an illuminator, for example (See JP-A-2000-280527, for example).
FIGS. 7 and 8 show an example of liquid crystal shutter. The liquid crystal shutter 9 shown in the figures includes a plurality of individual shutter portions 90R, 90G, 90B aligned in the primary scanning direction (the direction indicated by arrows A1, A2 in the figure). The liquid crystal shutter 9 includes a first and a second transparent substrates 91a and 91b arranged to face each other. Between the first and the second transparent substrate 91a and 91b, a rib spacer 97A is provided to locate at the periphery of the substrates. The rib spacer 97A, along with the first and the second transparent substrates 91a and 91b, defines a cell 96 for loading liquid crystal 90. The rib spacer 97a defines the height of the cell 96, i.e., the cell gap. In addition to the liquid crystal 90, spherical spacers 97b are filled in the cell 96. The spherical spacers 97b serve to stabilize the cell gap defined by the rib spacer 97a. 
The first transparent substrate 91a has a surface facing the second transparent substrate 91b and formed with a first transparent electrode 93a via an SiO2 film 92a. The SiO2 film 92a serves to enhance the adhesion of the first transparent electrode 93a to the first transparent substrate 91a. The first transparent electrode 93a is formed into an intended pattern by forming an ITO film and then etching the ITO film, for example.
The second transparent substrate 91b has a surface facing the first transparent substrate 91a and formed with a metal light-shielding film 94 having an opening 94a. The metal light-shielding film 94 allows light to pass selectively at the opening 94a. In the opening 94a are arranged color filters 98R, 98G and 98B for selectively passing red light, green light and blue light, respectively. The second transparent substrate 91b is further formed with a smoothing film 95 covering the color filters 98R, 98G and 98B, an SiO2 film 92b and a second transparent electrode 93b. 
The smoothing film 95 serves to compensate for a step formed by the provision of the color filters 98R, 98G, 98B for providing a smooth surface. The SiO2 film 92b serves to enhance the adhesion of the second transparent electrode 93b to the smoothing film 95. The second transparent electrode 93b partially overlaps the first transparent electrode 93a, and the overlapping portions constitute individual shutter portions 90R, 90B and 90B. Similarly to the first transparent electrode 93a, the second transparent electrodes 93b is formed into an intended pattern by forming an ITO film and then etching the ITO film, for example.
The second transparent substrate 91b is made larger in dimension than the first transparent substrate 92a. The second transparent electrode 93b extends over the second transparent substrate 92b up to a portion projecting outward relative to the first transparent substrate 92a. On the second transparent substrate 91b, a drive IC 99a is mounted for electrical connection to the second transparent electrode 92b. The drive IC 99a is connected to a flexible cable 99b via a signal electrode 99c. 
The liquid crystal shutter 9 has the following disadvantages due to the provision of the smoothing film 95 for covering the color filters 98R, 98G, 98B.
The smoothing film 95 is generally made of transparent resin and relatively soft. Therefore, spherical spacers 97b dispersed in the liquid crystal 90 may sink into the smoothing film 95 through the second transparent electrode 93b and the SiO2 film 92b. Such a phenomenon may occur at some locations in the cell 96, and the cell gap reduces at the locations where the spherical spacers 97b have sunk. Therefore, even when the same voltage is applied, the resulting electric field strength varies between a portion where the intended cell gap is maintained and a portion where the cell gap is reduced. As a result, the transmittance varies among the individual shutter portions 90R, 90G, 90B. The spherical spacers 97b are not dispersed evenly in the liquid crystal 90, and such unevenness of dispersion increases the variation of transmittance.
To achieve high-speed printing, the cell gap need be made relatively small for the purpose of driving the liquid crystal shutter 9 at high speed. However, when the cell gap is small, the influence of the unevenness of the cell gap due to the sinking of the spherical spacers 97b in the smoothing film 95 becomes relatively large. Therefore, in the liquid crystal shutter 9 having a relatively small cell gap, the variation of transmittance is large. In this point, the provision of the smoothing film 95 hinders the achievement of high speed printing.
Although the adhesion of the second transparent electrode 93b to the smoothing film 95 is enhanced by the SiO2 film 92b, the adhesion between the SiO2 film 92b and the smoothing film 95 is insufficient. Therefore, overetching is likely to occur in the etching process for forming the second transparent electrode 93b, so that the second transparent electrode 93b may become smaller than the intended pattern. In this case, the size of the individual shutter portions 90R, 90G, 90B differs between a portion where overetching has occurred and a portion where overetching has not occurred. At the individual shutter portion 90R, 90G, 90B corresponding to the portion where overetching has occurred, the numerical aperture becomes smaller, whereby the transmission efficiency at the shutter portion is reduced.
To reliably eliminate the step caused by the color filters 98R, 98G, 98B, the smoothing film 95 needs to have a relatively large thickness. In this case, a large amount of light is absorbed by the smoothing film 95, which further deteriorates the transmission efficiency.
To compensate for the deterioration of the transmission efficiency and to reliably irradiate the photosensitive film with a sufficient amount of light, the amount of light to be emitted from the illuminator need be increased, or the irradiation time for the photosensitive film need be increased. However, such measures are disadvantageous in terms of the running cost, and the increase of the irradiation time leads to the increase of the printing time.
Moreover, since the adhesion between the SiO2 film 92b and the smoothing film 95 is insufficient, when a stress is exerted on the interface between these films, the second transparent electrode 93b or the signal electrode 99c may be removed from the smoothing film 95 together with the SiO2 film 92b. Therefore, the mounting reliability of the drive IC 99a and the flexible cable 99b is deteriorated. In some cases, the mount surface of the second transparent electrode 93b or the signal electrode 99c may be physically rubbed for cleaning before mounting the drive IC 99a or the flexible cable 99b, or the drive IC 99a or the flexible cable 99b may be once removed for remounting. At that time, the second transparent electrode 93b or the signal electrode 99c may be removed, which hinders the mounting of the drive IC 99a or the flexible cable 99b. 
To solve the above problem, the smoothing film 95 should not be provided between the second transparent substrate 91b and the SiO2 film 92b at portions where the drive IC 99a and the flexible cable 99b are to be mounted. For this purpose, however, a patterning process to select the portions to form the smoothing film 95 need be added to the smoothing film formation step, which deteriorates the manufacturing efficiency and is disadvantageous in terms of the manufacturing cost.